Phenotypic effects of mosaicism for a 47,XXX cell line in Turner syndrome

Turner syndrome, gonadal dysgenesis with sex chromosome abnormalities, occurs in approximately 1/3000 liveborn females. Of females diagnosed with the condition, half are monosomic for the X chromosome. Among the rest, a multiplicity of chromosomal aberrations has been described. The more frequent are the presence of an isochromosome of the long arm of the X (i(Xq)) and ring X and mosaicism for two or more normal or abnormal cell lines (for example, 45,X/46,XX; 45,X/46,X,i(Xq); 45,X/46,XY). A small proportion (3-4%)1,2 of subjects with Turner syndrome are mosaic for a triple X (47,XXX) cell line. The triple X syndrome in the non-mosaic state is associated with a decrement in intelligence from that expected based on parental and sib accomplishment, normal stature, and normal fertility.3–8 Is the prognosis for females with Turner syndrome mosaic for a triple X cell line substantially different from that for females with 45,X? Does the presence of a third, normal 46,XX cell line in some of these females predictably affect phenotype? Is the risk for mental retardation or the likelihood of preserved fertility or normal stature greater in this category of people than in those with 45,X or 45,X/46,XX alone? I have reviewed our experience with 17 patients with Turner syndrome mosaic for a triple X cell line (11 with 45,X/47,XXX and six with 45,X/46,XX/47,XXX), and that of an additional 80 published case reports (18 with 45,X/47,XXX and 62 with 45,X/46,XX/47,XXX).1,9–64 These data are compared with those for the 227 girls and women with 45,X Turner syndrome and the 69 with 45,X/46,XX in our clinic database. Information gathered from medical records, direct examination, and self-report has been collected in a computer database as part of an ongoing, long term study of the natural history of Turner syndrome begun in 1977.2,65, …